The present invention relates to a laminar oriented thermoplastic film structure characterized by having a base layer and a heat sealable skin layer; and it also relates to a flexible multi-layer heat sealable general purpose packaging film.
In packaging applications using thermoplastic films, for example, polypropylene, it is usually necessary to seal the film during the packaging operation. This may be accomplished by using adhesives or by using heat sealing techniques. The particular heat sealing technique depends on the properties of the thermoplastic film, the packaging application, and the packaging technique being used.
In the packaging of certain types of foods, for example, loose products such as cookies, potato chips, and the like, it is common practice to employ a multi-layer film having two or more polymeric layers wherein one of the layers is known to be an effective heat seal layer. In the packaging process, a supply of such a multi-layer film can be shaped into a tube in a vertical form and fill machine. Marginal regions of the heat seal layer are brought into face to face relationship and heat sealed together. Thereafter, the packaging machine automatically forms a heat seal and makes a horizontal severance across the bottom of the bag; product is dispensed into the open end of the tube and, thereafter, a second horizontal seal is effected across the tube with a simultaneous severing through the tube to result in a product packaged in a tube, heat sealed at both ends and along one seam at right angles to the end seals. While the food or other product is being dispensed into the package, air is also present in the package and this air assists in protecting and cushioning the product during subsequent shipment of the finished packages. During shipment of the product, particularly with large size bags, e.g., those containing 16 ounces of product, the bags have a tendency to split or burst at the end seals.
The use of heat sealing techniques on oriented films may result in the loss of orientation in the vacinity of the heat seal. Heat sealed oriented films may, therefore, exhibit inferior and unacceptable film appearance and properties. In order to heat seal oriented films and obtain acceptable product properties, it may be necessary to apply to the oriented film a coating or laminar layer which has a melting point which is less than the orientation temperature of the oriented film. The application of the heat sealable laminar layer to the base oriented film may be accomplished utilizing various techniques including, melt extrusion of the heat sealable layer onto a preformed oriented base web employing adhesive material, or heat welding of the individual layers together. Preferred techniques are full extrusion of the individual layers followed by biaxial stretching of the multi-layer sheet so formed, or alternatively, extrusion coating of the heat sealable layer onto at least one surface of the longitudinally oriented substrate layer followed by stretching in the transverse direction.
In the past, coextruded oriented films have been prepared in which the oriented polypropylene core is coated with a skin layer comprising ethylene-propylene random copolymer containing minor amounts of ethylene. Such laminar films exhibit high heat seal strength but because of the relatively high melting point of the copolymer skin, i.e., in the order of from about 285.degree. F. to about 305.degree. F., the temperature range over which useable heat seal may be formed is quite narrow, i.e, about 10.degree.-20.degree. F.
Similarly, in the past, coextruded oriented films have been prepared wherein the core is an oriented polypropylene and the skin layers are comprised of low density polyethylene which has been polymerized utilizing conventional free radical-catalyzed processes. In the case of such laminates, because of the lower melting point of the low density polyethylene skin resin than that of the hereinabove-described ethylene-propylene copolymer skins, a broader sealing range is achieved. However, the seal strength provided by a skin comprised of such polyethylene homopolymers, polymerized by a free radical-catalyzed process, are not as high as those provided by an ethylene-propylene random copolymer skin. Moreover, the haze and gloss values of such films are poorer than the haze and gloss values exhibited by oriented polypropylene film.
As indicated above, in packaging applications utilizing polypropylene based film it is usually necessary to seal the film during the packaging operation. A multi-layered wrapping film of this type is described in U.S. Pat. No. 4,214,039, the subject matter of which is, in its entirety, incorporated by reference herein. This patent describes a packaging film structure comprising a polypropylene film substrate having a heat-sealable vinylidene chloride polymer containing at least 50% by weight of vinylidene chloride. In between the polypropylene film and the vinylidene chloride polymer is a primer coat which consists of the reaction product of an acidified aminoethylated vinyl polymer and an epoxy resin. This primer coat enhances the bond strength between the propylene and the heat sealable vinylidene chloride polymer. While this packaging material is effective for the packaging of comparatively small quantities of a product, there exists a need to increase its seal strength when used for the packaging of comparatively large quantities of product. Particularly, there is a need to increase the seal strength between the polypropylene layer and the primer layer.